tnef: fix gcc-15 build failure

Without the change the build fails on `master` as
https://hydra.nixos.org/build/318200761:

```
gcc -DHAVE_CONFIG_H -I.     -g -O2 -c -o attr.o attr.c
attr.c:242:1: error: conflicting types for 'attr_read'; have 'Attr *(FILE *)'
  242 | attr_read (FILE* in)
      | ^~~~~~~~~
In file included from attr.c:29:
attr.h:72:14: note: previous declaration of 'attr_read' with type 'Attr *(void)'
   72 | extern Attr* attr_read ();
      |              ^~~~~~~~~
```

pkgs/by-name/tn/tnef/package.nix

@@ -2,6 +2,7 @@
   lib,
   stdenv,
   fetchFromGitHub,
+  fetchpatch,
   autoreconfHook,
 }:
 
@@ -16,6 +17,15 @@ stdenv.mkDerivation rec {
     sha256 = "104g48mcm00bgiyzas2vf86331w7bnw7h3bc11ib4lp7rz6zqfck";
   };
 
+  patches = [
+    # Fix gcc-15 build failure: https://github.com/verdammelt/tnef/pull/49
+    (fetchpatch {
+      name = "gcc-15.patch";
+      url = "https://github.com/verdammelt/tnef/commit/86bfa75cfacbe71c8d5282fa0065981b4544c5ad.patch";
+      hash = "sha256-iWQop57riqwDLVi5Ba5s4f34lGXgvKO3ZMTgWbAoRIY=";
+    })
+  ];
+
   doCheck = true;
 
   nativeBuildInputs = [ autoreconfHook ];